The present invention relates generally to wireless communications, and more specifically to power saving methods for wireless devices.
The IEEE 802.11e task group (TGe) is defining enhancements to the base IEEE 802.11 standard for Quality-of-Service (QoS). TGe has recently adopted an 802.11e draft standard that attempts to extend QoS to power-save stations. The current 802.11e power-save methods are not uniform and suffer from the issues which will be discussed below.
An Overview of 802.11e Power-Save Mechanisms:
The current 802.11e draft augments the PS-Poll power save mechanism with two new power-save methods:
1) The Automatic Delivery Power-Save (APSD) method. A Quality of Service wireless station (QSTA) uses the current 802.11e APSD mechanism to establish Wakeup Beacons, where the QSTA automatically transitions to an awake state and the access point (AP automatically delivers buffered downlink frames to the QSTA, following each Wakeup Beacon. The APSD mechanism is an extension of the CF-Pollable power-save mechanism in the base 802.11 standard.
2) The “Service Schedule” method. With the 802.11e Service Schedule method, a QSTA uses traffic specification (TSPEC) signaling to establish QoS service requirements. The Hybrid Coordinator (HC) in the access point (AP) aggregates the TSPEC information and establishes periodic Service Periods for the QSTA by sending a Schedule element to the QSTA. A QSTA must be awake for the start of each Service Period.
In addition, the 802.11 base standard defines a power-save mechanism, however, this power-save mechanism is not considered to be suitable for QoS applications.
The APSD method is very useful for asynchronous applications and applications that are not latency sensitive. However, for applications such as Voice over Internet Protocol (VoIP), APSD has the following concerns:
1) APSD requires a very fast Beacon rate to support a typical VoIP sampling rate;
2) APSD tends to crowd downlink data around Beacons; therefore, QSTAs must often remain awake while frames are transmitted to other QSTAs. It should be noted that 802.11 Beacons contain a Traffic Indication Message(TIM), so that QSTAs can receive both Beacons and downlink data in the same wakeup interval. A QSTA can immediately go back to a Doze (power-save) state if its TIM bit is set OFF; otherwise, it must stay awake to receive downlink frames buffered in the AP.
3) The APSD method adds “latency” to downlink power-save transmissions, because downlink frames are delayed until the next Wakeup Beacon.
The current 802.11e Service Schedule methods has the following concerns:
1) A “Service Period,” as defined in the current 802.11e draft, starts with the first successful downlink transmission. If there is not an uplink transmission in each Service Period, then Service Periods can become unsynchronized such that the AP and QSTA disagree on the next Service Period start time. One proposed solution is to require at least one uplink data frame in each Service Period, however, such a solution is not desirable because it adds extra traffic.
2) The timer logic required for Service Period scheduling is complex and different than the timer logic required for the APSD mechanism.
3) The Service Period mechanism adds latency to both uplink and downlink transmissions, because transmissions are delayed until the next Service Period.
Thus, the need exists for an efficient power-save method suitable for QoS applications.